A diet high in fat is considered to be a risk factor for obestiy, heart disease, and for cancer of the breast, prostate, and colon. Numerous studies are in progress to examine the influence of dietary fat on the occurrence and course of these diseases. At present, measurements of dietary fat intake in these studies rely on the memory of study subjects or their ability to self-record their normal diet. No known biochemical markers of dietary fat intake exist which could be used to objectively validate such measurements. For this reason we were stimulated to search the literature and evaluate potential markers. Subsequently, we have decided that the relationship between dietary fat intake and the plasma esterified to nonesterified carnitine (EC/NEC) ratio warrants further investigation. This study is designed to test the hypotheses that the difference observed by other researchers in the plasma EC/NEC ratio of subjects fed different fat levels is significant and maintained over time, and that sudden increases or decreases in dietary fat intake do not cause corresponding sudden increases or decreases in the plasma EC/NEC ratio, but that there is a residual effect of consuming a high fat diet. Twenty female, nonhuman primates, 13 to 18 years old, which have been accustomed to a high fat diet (HFD), containing 40% of the total calories as fat, will be randomly assigned to either a control or study group. The ten animals in the control group will be maintained on the HFD for the duration of the trial. The ten animals in the study group will be switched to a low fat diet (LFD), containing 20% of the total calories as fat, for the first 14 weeks, and then will be switched back to the HFD for the remaining 12 weeks of the trial. Blood samples will be taken from both groups for analysis of the plasma EC/NEC ratio as follows: 1) Twice during the week prior to switching the study group to the LFD, in order to establish baseline values; 2) approximately every three days during the first three weeks following dietary changes; and 3) one per week at all other times. Differences in the plasma EC/NEC ratio between study and control group, and differences over time within groups will be analyzed. If the above hypotheses are proven correct in these primates, it will set the stage for further research to determine the efficacy of using the plasma EC/NEC ratio as a biochemical marker of dietary fat intake in humans.